Fiber grating coupled quasi-monochromatic light sources capable of tunable single frequency operation are highly desirable. One area of application that illustrates the need for such capability is spectroscopy. Although not all spectroscopic applications require single frequency operation, the narrower the absorption line of a molecular or atomic species, a test subject, the greater the need that the spectral width of an output of a light source be narrow to detect the absorption line.
In spectroscopy, the amount of light absorbed by a test subject is recorded as a function of wavenumber, the frequency of a wave divided by its velocity of propagation (the reciprocal of wavelength) resulting in a lineshape or absorption line. Spectroscopy is often used to identify substances by their known absorption lines. Absorption lines vary with environmental parameters. For example, a decrease in pressure will narrow the absorption line of a known species such as water vapor. In spectroscopic applications, light source wavelength, tuning range and spectral quality are important.
A light source whose wavelength may be tuned over a wide range is desirable so that it may be used to detect absorption lines at a broader range of wavelengths. For some applications, single frequency operation is highly desirable, if not required, in order to detect substances with narrow bandwidth absorption lines. Such an application would be measuring water vapor on Mars, in particular obtaining short range, meaning a few kilometers, resolved planetary atmospheric profiles using an active spectrometer at the heart of a water vapor lidar instrument. There is no atmospheric pressure on Mars. This causes the absorption line of water vapor on Mars to be within a much narrower bandwidth than on Earth where the atmospheric pressure tends to spread the absorption line of water vapor. A spectrometer capable of single frequency operation is desirable because the narrow line of the light source of such a spectrometer can be tuned to within the narrower band of wavenumbers comprising the absorption line of water vapor on Mars and detect it. Without single frequency operation, the spectrum of the light output of the spectrometer may be spread too broadly to accurately detect the presence of water vapor resulting in an inaccurate atmospheric water vapor profile on Mars.
Compactness, low power consumption, high accuracy of determining constituent content, and low-cost are all desirable features in a spectrometer also. Laser based spectrometers have used tunable dye lasers, titanium sapphire lasers, and laser diodes as their light sources. Tunable dye lasers and titanium sapphire lasers are typically bulky, inefficient and expensive. Advanced manufacturing machinery for epitaxial growth has enabled the mass production of semiconductor lasers and light emitting diodes (LEDs) throughout the visible and near infrared portion of the optical spectrum. These devices have proved to be low-cost, operable at room temperature, reliable devices providing a diversity of available wavelengths. However, previous use of diode lasers in active spectrometers without fiber grating feedback have shown the difficulty in obtaining single frequency operation due to the multimode operation of laser diodes and their temperature sensitivity causing wavelength instability. The multimode behavior of diode lasers such as Fabry-Perot diode lasers in the GaAs family is not conducive to stability and wavelength reproducibility desirable in a spectrometer. Moreover, laser diodes emit a rapidly diverging beam which produces an astigmatism in its light output. As a consequence of the astigmatism, coupling geometries that accommodate the transverse numerical aperture of the laser are defocused in the lateral dimension, even for aspherical optics. The mismatch produces a quadratic phase variation in the feedback along the lateral axis at the facet of the laser that excites lateral modes of higher order than the single longitudinal mode, TM.sub.00.
To obtain single frequency operation, two types of diode lasers with a grating incorporated in the semiconductor structure have been developed. One such type is the Distributed Feedback (DFB) laser which has the grating adjacent to the active region of the diode where current flows. The manufacturing process of this type of laser results in narrower wavelength dispersion than in multimode Fabry-Perot lasers. Another type is the Distributed Bragg Reflection (DBR) lasers in which the grating is in the passive part of its cavity where no current flows. The fact that the grating is incorporated in the semiconductor structure requires custom manufacturing for specific applications. Custom wavelength DFB or DBR lasers are still very expensive, costing more than $100,000. Furthermore, single frequency operation is still limited in these lasers due to sensitivity to temperature fluctuations.
Commercial laser diodes have been placed in external cavities with bulk gratings enabling a wide tuning range and wavelength control. Bulk grating external cavity laser diode systems have been optimized and are commercially available. The drawbacks of these systems include that they are still reasonably expensive, greater than five thousand dollars ($5000), and are physically too large to incorporate into many possible tunable diode laser based products in which compactness is highly desirable.
As illustrated by the spectroscopy example, low-cost fiber grating coupled light sources, such as light emitting diodes, Fabry-Perot laser diodes, and optical amplifiers, capable of single frequency operation, or at least operation within a narrow optical spectral width of the desired wavelength are highly desirable.